This application requests support for 13th International Workshop on Radiation Damage to DNA to be held at the Massachusetts Institute of Technology in Cambridge, MA, from June 14 through June 18, 2014. This will be the thirteenth in a highly successful series of uniquely interdisciplinary workshops that bring together the leading experts in physics, chemistry, biology and medicine who have an interest in understanding the effects of ionizing radiation on DNA and in translating the research ideas to clinical and public health outcomes in prevention, diagnosis and therapy. The participants, limited to 150, will be from diverse academic and research institutions and will include graduate students, postdoctoral fellows, and early-career and senior scientists. A unique feature of this program is its breadth of topics ranging from physical energy deposition of ionizing radiation in DNA through to the ultimate biological outcome. In 15 sessions with two Keynote Lectures and two poster sessions, the Workshop introduces new topics such as the biology of space radiation, the epigenetics of radiation biology, synthetic lethality in radiotherapy, therapeutic radiomitigators and radiosensitization by high-Z nanoparticles, and the biological impact of low-dose radiation. The Workshop also addresses the latest developments in the physics and modeling of radiation tracks, molecular damage and DNA repair; the chemistry of radiation-induced DNA oxidation; the chemical mechanisms and biological consequences of clustered DNA damage and damage to chromatin; repair of radiation-induced DNA double-strand breaks and clustered lesions; new tools for studying DNA repair in real time; and a memorial symposium to honor a pioneer in radiation chemical biology, Clemens von Sonntag. The intimacy of the Workshop format promotes discussion of the mechanisms involved in the production of DNA lesions, the ultimate biological consequences of those lesions, and pathways to disease prevention, diagnosis and therapy. Although the focus spans physics, chemistry and biology, an important goal of this meeting is to decrease polarization by discipline and stimulate new, interdisciplinary thinking and experimental approaches. Along with invited presentations and poster sessions organized to promote formal and informal discussions, the Workshop structure emphasizes the participation of young investigators with an oral presentation in each session for graduate students, postdoctoral fellows and junior faculty, and encourages their professional development by mentoring and networking with senior researchers.